A hemming of bending a flange erecting from an edge of a panel inward of the panel may be performed on edges of a bonnet, a trunk, doors, and wheel housings of an automobile. As the hemming, a roll hemming of positioning and holding the panel on a fixed die, and bending the flange at the end of the panel while pressing a roller against the flange can be exemplified. Since the bending angle is large in such roll hemming, working may be performed through a plurality of processes, including preliminary bending (or pre-hemming) and finish bending (or main hemming) in consideration of bending precision.
As such roll hemming, a method of setting a workpiece in a die provided for an exclusive process in an exclusive space, and rolling a unit held at a tip of a robot along the flange to perform roll hemming is suggested (for example, refer to JP-Y2-2561596 and JP-B2-2924569). In the method, working is performed with a workpiece placed on the upper surface of a large fixed die.
Further, hemming a rim strip of an end of a workpiece such that the rim strip is pinched and is pressed by a pressing roller while a pressure roller is rolled on a thin and elongate protection strip corresponding to a die in a state where the protection strip is applied to the rim strip is suggested in a flanging apparatus described in JP-A-2006-110628.
In the method of placing a workpiece on the upper surface of a fixed die to perform roll hemming, a fixed die that supports the whole workpiece is needed. Therefore, if the workpiece is large, it is also necessary to make the fixed die large-sized in accordance with the workpiece. Especially, even in a case where roll hemming is performed only on a portion of a workpiece, the fixed die should support not only a spot to be worked but also the whole workpiece. Therefore, a large-sized fixed die is required, which is not rational. Further, when various kinds of roll hemming are required for every portion to be worked, a plurality of corresponding fixed dies are provided, and consequently, keeping and management of the dies are complicated.
Moreover, in the above method, the arrangement and configuration of other peripheral devices are regulated on the basis of a fixed die. As a result, it is necessary to provide an exclusive space and an exclusive process for roll hemming, and it is difficult to provide the exclusive space and process on a normal production line. Accordingly, it is necessary to convey a workpiece between other assembling and working processes, and the workpiece to be applied is limited to a small one that can be conveyed. That is, it is difficult to apply roll hemming to a large workpiece after assembling, and there is a restriction that roll hemming should be performed on every small part before assembling.
From such viewpoints, if roll hemming is performed on a wheel arch, etc. in a production process of automobiles, a sheet metal will be assembled into a white body after roll hemming is performed on the sheet metal in the vicinity of the wheel arch in an exclusive hemming process. As a result, productivity improvements are further desired from viewpoints, such as an output space, conveyance between processes, and assembling time.
On the other hand, in a method described in JP-Y2-2561596, the posture during preliminary bending of a roller unit (FIG. 2 in JP-Y2-2561596) and the posture during bending (FIG. 5 in JP-Y2-2561596) differ greatly. Therefore, time is required for transition between these postures, and the control procedure of the postures is complicated. Moreover, it is difficult to regulate the posture and pressing force of a hemming roller during the preliminary bending, and as shown in FIG. 15, there is a probability that a flange 900 may be unnaturally bent, like being excessively bent or undulated.
Further, in a method described in JP-B2-2924569, a guiding groove is only one. Therefore, different hemming rollers should be used during the preliminary bending and finish bending, and extra time is required for roller replacement. Moreover, since the guiding groove is provided in the front face of the die, the force to be applied to the flange during finishing (FIG. 3C in JP-B2-2924569) may also be distributed to a guide roller that is engaged with the groove, and since the groove serves as a stopper, the force to be applied may be limited.
In the method described in JP-A-2006-110628, the protection strip corresponding to a die cannot be automatically mounted on a workpiece, and a worker should mount separately and manually. However, since a fairly large-sized framework or clamp, etc. is provided in the protection strip so as to mount the protection strip to the workpiece, it is heavy and complicated. Accordingly, there is the same disadvantage as the fixed die in that it is difficult to provide the frame work or clamp on a normal production line.
Further, in JP-A-2006-110628, the pressure roller has a general cylindrical shape, and the protection strip that is in contact with the pressure roller also has a smooth surface. Thus, since the pressure roller and the protection strip cannot be positioned mutually, exact rolling in a desirable direction cannot be made.
Moreover, JP-A-2006-110628 discloses an example in which a sensor roller serving as a third roller is provided so as to position the pressure roller with respect to the protection strip. In this case, the pressure roller is in contact with a side face of the protection strip, and the sensor roller is in contact with one end face of the protection strip. In such a method, since there is the sensor roller on one end face of the protection strip, positioning is made in this direction. However, since there is no positioning means on the other end face, positioning is not made in the opposite direction, but deviation will occur. Further, since three rollers are needed, structure is complicated.